There exists a class of machinery which utilizes mechanical resonance as the means to obtain periodic motion of the machine elements. Reciprocating compressors of this class, often referred to as "resonant piston compressors," can be advantageously used in a variety of applications, such as for example, electrically-driven heat pump systems and the like.
In known free-piston resonant reciprocating compressors the fluid compressing member, such as a piston, is driven by a suitable motor, such as a linear reciprocating electrodynamic motor. A compression piston is usually coupled to the motor armature and the armature held in a rest position by way of one or more main or resonance springs. When the motor is energized, such as by an alternating current, a periodic magnetic force is generated to drive the piston. If the frequency of the periodic magnetic force is sufficiently close to the mechanical resonance frequency of the compressor (as determined essentially by the mass of the reciprocating assembly and the combined stiffness of all mechanical and gas spring components), the piston will oscillate back and forth to provide compression of the fluid.
U.S. Pat. Nos. 3,937,600 to White for a "Controlled Electrodynamic Linear Compressor" and 4,353,220 to Curwen for a "Resonant Piston Compressor Having Improved Stroke Control for Load-Following Electric Heat Pumps and the Like" relate to double-ended type, electrodynamic motor-driven reciprocating compressors including gas springs. In such double-ended two-compressor-cylinder arrangements, identical parallel flow cylinders are involved. In principal, these two cylinders would undergo the same compression cycle and would be subjected to the same pressure forces so that such double-ended design would (in theory) be intrinsically pressure balanced. In practicality, however, such designs are inherently unstable. As long as the two cylinders operate with the same value of mid-stroke volume (or equivalently, at the same clearance volume ratio) then the two cylinders will impose equal but oppositely-directed (cancelling) average pressure forces on the plunger-driven pistons. However, any slight offset bias of the plunger from the theoretical center position causes the average pressure forces on the two pistons to be unbalanced in such a way that it tends to push the plunger further off center, resulting in an axially unstable arrangement. To solve such a situation, these patents introduce ports on the gas springs. When the piston begins to go off center, an opposing average pressure force which is larger than the destabilizing force coming from the cylinder would be generated resulting in a stable operating center position.
While such an arrangement has proved eminently satisfactory in the two-compressor-cylinder arrangement, axial positioning stability in a single cylinder arrangement is also desired.